The next-generation multimedia wireless communication systems, which have recently been actively studied, are required to process a variety of information such as video and wireless data at a higher data rate beyond the early voice-oriented service.
To this end, the wireless communication systems use a power control scheme as one method for reducing path losses associated with Base Stations (BSs) and their terminals and also reducing inter-cell interference caused by interference from adjacent cells. The power control scheme is a scheme that adjusts transmission power to transmit data with the lowest possible power level while maintaining Quality of Service (QoS) of the wireless communication systems at a threshold or above.
Meanwhile, if uplink transmission power is low, BSs cannot receive transmission signals from terminals. In contrast, if uplink transmission power is high, transmissions signals of terminals may cause interference to transmission signals of other terminals, and battery consumptions of the terminals also increase.
If the uplink transmission power is controlled, strengths of received signals are maintained at an optimal level, making it possible to prevent the unnecessary power consumption in terminals. In addition, if the uplink transmission power is controlled, data rates are adaptively determined, contributing to improvement of transmission efficiency.
As noted above, uplink performance of the wireless communication systems is determined based on the transmission power of terminals.
To control transmission power, wireless communication systems use two different approaches: an open-loop power control scheme and a closed-loop power control scheme. First, the open-loop power control scheme measures or estimates downlink signal attenuation to predict uplink signal attenuation, and compensates uplink transmission power based on the predicted uplink signal attenuation. The closed-loop power control scheme determines uplink transmission power based on the properties of transmission data or the amount of wireless resources allocated to a particular terminal.
The closed-loop power control scheme adjusts uplink transmission power using feedback information for transmission power control, which is exchanged between BSs and terminals.
The most important thing in determining the uplink transmission power is to determine a level of uplink transmission power that is set for each terminal. The uplink transmission power, which is set for each terminal, is determined mainly depending on the level of compensating for a path loss between the terminal and its BS. A method for determining uplink transmission power includes, for example, Fractional Power Control (FPC) proposed by Long Term Evolution (LTE).
FIG. 1 illustrates a general wireless communication system for controlling uplink transmission power.
The wireless communication system includes a terminal 115, a serving BS 110 communicating with the terminal 115, adjacent BSs 130a-130f, and terminals 135a-135f communicating with the adjacent BSs 130a-130f, respectively.
The terminal 115 determines a strength of transmission power based on information about transmission power received from the serving BS 110 and transmits data using the transmission power of the determined strength. The signal transmitted by the terminal 115 is affected by the interference of signals transmitted by the respective terminals 135a-135f communicating with the adjacent BSs 130a-130f. In addition, based on the scheduling policy of each BS, terminals transmitting signals are changed in every allocation of resources such as time and frequency.
As stated above, the conventional uplink transmission power control method considers only the path loss between the serving BS and its terminal, but does not consider the interference affecting the adjacent cells. Hence, there is a need to optimize the overall system performance by considering not only the path loss between the serving BS and the terminal, but also the interference affecting the adjacent cells, during uplink transmission power control.